PARADOXICAL VARIATION OF STRENGTH DETERMINANTS WITH DIFFERENT ROTATION AXES IN TRUNK FLEXION AND EXTENSION STRENGTH TESTS

The aim of this study was to illustrate the influence of different lev els of the fulcrum (the axis of sagittal rotation) on measured trunk f lexion and extension strength and compare force and torque as a unit o f measure. The isometric trunk strength was measured in 16 healthy fem ale subjects. The dynamometer was kept at the shoulder level and the m oment arm was lengthened step by step by moving the fulcrum caudally f rom the level of the posterior superior iliac spine to the level of th e gluteal fold. The moment of force (torque) increased from 117.0 to 2 08.5 N.m in flexion and from 182.2 to 292.5 N.m in extension, P<0.0001 . An attempt to quantify this change was made. Paradoxically, the meas ured force remained at a constant level (in flexion) or slightly decre ased (in extension). We concluded that torque as a measure of trunk fl exion and extension strength is highly dependent on the level of the r otation axis and force appears to be less sensitive for variations wit h the height of the fulcrum. We would suggest that the observed increa se in torque is physiological and reflects to what extent hip flexor o r extensor muscles are recruited. The force, on the other hand, may be tter characterize a person's capability to perform functional tasks. F orce and torque should strictly be distinguished from one another.